As evidenced by this post, they are not a monopoly. So what does this statement actually mean?
This post gives evidence of the contrary, game that dared not to be published on platform that has 70-80% of pc market share ( https://www.geekwire.com/2024/gaming-giant-valve-hit-with-another-antitrust-lawsuit-alleging-anticompetitive-practices/ ) shouldn't even exist and even stating otherwise is a blasphemy, lol. As for more anecdotical examples, games that are published on steam only, are majority most don't even list other platforms on their web sites, in cases when they can be bought elsewhere. Even more, updates and patches often do not reach other marketplaces. So yeah steam is a shitty marketplace with horrible ui and captured fanatical clients.
It came out okay-ish, no real game-breaking bugs, after patches it is even better.